1. Field of the Invention
The present invention relates generally to FSK data waveform shaping circuits, and more particularly, to a waveform shaping circuit for use in a FSK data receiving system.
2. Description of the Background Art
The FSK data receiving system is for receiving FSK (Frequency Shift Keying) modulated data and it is used, for example, as the paging system. The paging system is a system wherein the determination is made whether or not received FSK data includes predetermined data, to inform a person of the detection of the FSK data including the predetermined data by a buzzer or the like.
FIG. 1 is a block diagram showing a general arrangement of a FSK data receiving system for use as such paging system as described above or the like. In the drawing, the FSK data receiving system comprises a radio frequency amplifier circuit 31, a frequency converting circuit 32, an intermediate frequency amplifier circuit 33, an FM detecting circuit 34, a low pass filter 35, a waveform shaping circuit 36 and a CPU 37. The CPU 37 has the functions of FSK demodulation by discriminating a width of output pulse of the waveform shaping circuit 36, controls of a receiving block based on the demodulated signal and calling or displaying.
In the FSK data receiving system structured as described above, a direct current level of detection output of the FM detecting circuit 34 fluctuates due to fluctuations of a local oscillating frequency of the frequency converting circuit 32 and variations of characteristics of the FM detecting circuit 34 and the like. In the FSK demodulation, demodulation by discriminating a pulse width, because the data has the direct current level, it is impossible to perform fidelity demodulation by a waveform shaping circuit simply comparing the detection output with a fixed reference potential. Therefore, such waveform shaping circuit as shown in FIG. 2 is conventionally used in common.
In FIG. 2, a terminal 41 receives an FM detecting signal V.sub.DET through the low pass filter 35 of FIG. 1. A time constant circuit comprising a resistance R.sub.41 and a capacitor C.sub.41 is connected between the terminal 41 and a non-inversion input terminal of a comparator 42. Since a frequency of a signal to be used in the FSK system is several hundreds Hz, a time constant of the time constant circuit is set to be relatively large. In addition, a resistance R.sub.42 of the same resistance value as that of the resistance R.sub.41 is connected between the terminal 41 and an inversion input terminal of the comparator 42 to prevent offset. Furthermore, a power supply terminal of the comparator 42 is grounded through a control transistor Q.sub.41. A battery save signal (referred to simply as a control signal V.sub.BS which intermittently attains a high level is inputted to a base of the control transistor Q.sub.41. Consequently, the waveform shaping circuit 36 intermittently operates, which reduces power consumption. The control signal V.sub.BS is also supplied to other circuit blocks of the FSK data receiving system.
Now, referring to a waveform diagram of FIG. 3, an operation of the conventional waveform shaping circuit shown in FIG. 2 will be described. As shown in FIG. 3, the control signal V.sub.BS is an intermittent signal attaining a high level in the periods I and II and a low level in the period III. When the control signal V.sub.BS attains the high level, the comparator 42 is brought into operation and the other circuit blocks start operating, so that the FM detecting signal V.sub.DET is inputted to the terminal 41. Then, in the period II after the lapse of about .tau. (=C.sub.41 R.sub.41) from the rise of the control signal V.sub.BS, the time constant circuit inputs a mean value voltage V.sub.AV of the FM detecting signal V.sub.DET to the non-inversion input terminal of the comparator 42. In the period III wherein the control signal V.sub.BS attains the low level after the period II, charged electric charges in the capacitor C.sub.41 of the time constant circuit are discharged through the resistance R.sub.41 and the low pass filter 35. Accordingly, out of the respective periods wherein the control signal V.sub.BS attains the high level, the comparator 42 performs accurate waveform shaping only in the period II. For the purpose of simplicity, the waveform of the FM detecting signal V.sub.DET of FIG. 3 is demodulation of the FSK data signal modulated by the data simply cycling "1010 . . . ".
As the foregoing, in the conventional waveform shaping circuit shown in FIG. 2, the charged electric charges in the capacitor C.sub.41 of the time constant circuit are discharged every time the idle period III comes as shown in FIG. 3. Therefore, even if the control signal V.sub.BS attains the high level, the normal waveform shaping operation can not be performed in the period I, whereby the power consumption during the period will be a wast. Accordingly, the electric power can not be made full use of. In addition, since the time constant circuit has a time constant of about 200 ms, it can not respond to transient fluctuations in a direct current level, causing a frequency to shift, which results in that accurate waveform shaping can not be performed.